Hyperthermia is a relatively new area in the medical sciences for treating subsurface cancerous or malignant tissues, such as tumors. It has become well known that the destruction of such cancerous tissues can be accelerated by elevating the temperature thereof to about 43.degree. C. When the malignant tissue is elevated to such temperature for a period of time, for example, an hour, the malignant cells are weakened and are eventually destroyed. Hyperthermia treatment is generally carried out in conjunction with radiation treatments to destroy malignant tissues, especially those in which the tissue is in a relatively well defined area below the surface of the skin.
The mechanism which allows the cancerous tissues to be destroyed, without destroying normal surrounding tissue, is that the malignant tissue is generally characterized by poor blood circulation with reduced oxygen. The microwave energy induced within the tumorous tissue heats the tissue to such an extent that the limited blood circulation cannot dissipate the heat. This contrasts with normal healthy surrounding tissue having sufficient blood circulation which readily carries the microwave-generated heat away. For a better understanding of hyperthermia treatment techniques and apparatus, reference is made to U.S. Pat. Nos. 4,397,313; 4,397,314 and 4,446,874,all assigned to Clini-Therm Corporation, Dallas, Tex.
While there exists many different techniques for elevating the temperature of subsurface malignant tissues, a common concern attendant with all such techniques is that of the uniform heating of the tissue to assure that the well-defined temperature of 43.degree. C. is achieved throughout the malignant tissue. In other words, in many microwave application techniques where the heat distribution, or temperature gradient, within the malignant tissue is non-uniform, hot spots and cool spots will occur, thereby reducing the effectiveness of the treatment.
Microwave applicators utilized for the hyperthermia treatment of tissue include interstitial antennas which are inserted into the malignant tissue. A plurality of such antennas, generally being of a quarter wavelength with respect to the transmitting frequency, are inserted into the patient in a matrix pattern in an attempt to achieve uniform heating. Not only is this painful to the patient, but also the lateral dipole radiation is accompanied with cool spots which exist between the dipole radiation patterns. Cavity or waveguide type microwave applicators held against the skin of a person are not an invasive type of applicator, but are heavy and bulky and require special apparatus for holding against the patient during the long periods of microwave application. More recently, there has been developed printed microstrip circuit boards with a pattern of antennas, each about 10-14 wavelengths long. This latter approach is reported to be effective, but can cover only a small portion of the surface area of the applicator. Thus, additional robotics equipment is utilized to continuously move the small microstrip applicator about the surface of the tumor so that over a period of time the entire malignant tissue is heated to the desired temperature. It is apparent that in utilizing this approach a higher instantaneous power is required to achieve the desired temperature of the malignant tissue.
From the foregoing, it can be seen that a need exists for a new microwave application technique, and corresponding apparatus, for providing a non-invasive and highly uniform application of microwave energy to a subsurface malignant tissue. An additional need exists for microwave energy applicators which are lightweight and easily constructed with different surface areas to accommodate different sizes of malignant tumors. Yet another need exists for a microwave applicator which can be utilized with currently available microwave transmitters, and which requires no additional skill on the part of the personnel administering the hyperthermia treatment.